1. Field of the Invention
The present invention relates to the art of powder metallurgy, and more particularly to a method of and an apparatus for pressing a fine metal powder into a compacted body having a gear shape or the like which will be sintered into a gear as a powder metallurgy product, in a manner to increase the mechanical strength of a local region of the completed gear.
2. Description of the Related Art
There has heretofore been known a powder metallurgy process for manufacturing sintered mechanical parts such as gears which are required to have a desired degree of wear resistance and rigidity. According to the powder metallurgy process, a fine metal powder is pressed into a compacted body having a gear shape, for example, by a pressing machine, and the compacted body is heated, i.e., sintered, into a gear. The pressing machine comprises a lower die having a cavity for receiving the fine metal powder and an upper punch movable into the cavity to press the fine metal powder into a compacted gear shape.
For producing a gear with a hole defined centrally therein using such a pressing machine, a fine metal powder is filled in the cavity of the die around a core placed at the center of the cavity, and then the core is lifted to a position where the upper surface of the core is higher than the upper surface of the filled fine metal powder. Thereafter, the punch is lowered into the cavity to press the fine metal powder around the core. The compacted body has a hole defined centrally therein by the core which has been removed.
Heretofore, powder metallurgy products that are pressed by a die and a punch have uniform mechanical properties because they are subject to uniform loads over their entire surface regions. Some mechanical parts need to have different mechanical properties in different regions thereof. For example, gears should have greater strength and toughness at their teeth and greater wear resistance at their center. However, it has been impossible for powder metallurgy products to have different local mechanical properties.
FIG. 13 of the accompanying drawings shows a conventional process of manufacturing a pressed powder body as a gear blank. As shown in FIG. 13, when a core 51 is lifted to a position where it is higher than the upper surface of a fine metal powder G placed in a die cavity, the circumferential edge of the upper surface of the core 51 scrapes off and carries upwardly a surrounding layer of the metal powder G, resulting in a reduction in the density of the fine metal powder G around the hole therein. After the fine metal powder G thus shaped is pressed by an upper punch 52 and then sintered into a gear, the formed gear has a reduced mechanical strength around its central hole. As a consequence, if a rotational shaft is pressed into the central hole of the gear, then the region of the gear around the central hole tends to be damaged in use especially if the gear is a helical gear or the like which is likely to develop eccentric stresses in the central region of the gear. If the gear is machined to form internal gear teeth around its central hole, then the internal gear teeth are apt to be broken in use.
There has been a demand in the art for a process of preventing the density of a fine metal powder from being lowered around a hole which is defined in the fine metal powder by the core 51, so that the mechanical strength of the completed gear will not be reduced around the central hole thereof. Stated otherwise, it has been desired in the art of powder metallurgy to process a fine metal powder, at the time of pressing and sintering it into a powder metallurgy product such as a gear, in a manner to give different local mechanical properties to the completed powder metallurgy product.